The easiest way to change a mind forever is to destroy bits of the brain. It's not very precise, but it is remarkably effective. This has been known ever since Phineas Gage, an enthusiastic railway worker, detonated the charge of dynamite he was tamping into a hole, so that the spike he was tamping it with flew out, smashed his cheekbone and burst through his brain and out the top of his skull. He lived for years after that but he had lost almost all his inhibitions. They had somehow been contained in the part of his brain that was destroyed.

This story is known to everyone interested in the relationship between mind and brain. But there is one strange and horrifying pendant which I only learned last week, at a seminar in Cambridge. Brain injuries of a certain sort can disinhibit adults. In children they can permanently prevent the formation of inhibitions at all. Such things are fortunately very rare. But they are recorded, especially at the university of Iowa, which collects patients from all over the state and thus as an unmatched, unenviable knowledge of ghastly childhood brain injuries, whether from cancers, epilepsies or simple accidents.

The classic paper on this is more than ten years old: in 1999 Antonio Damasio and colleagues at the University of Iowa published a study of two young adults who simply did not live in the same moral universe as the rest of us. One of them was a girl who had been run over as a toddler. She appeared to make a quick and full recovery. But as an adolescent, she became more or less psychopathic. Although intelligent and academically capable, she stole, she fought, she lied chronically; at 18 she had a child whom she neglected. Most tellingly of all, she could not see anything wrong with her behaviour, nor even pretend that she did.

Their other case was a boy who had had a brain tumour as a baby, which was removed when he was three months old. Again, there were no obvious, immediate signs of trouble, but as he grew up he became incapable of satisfying anything except his immediate, grossest appetites.

Left to himself, he limited his activities to viewing television and listening to music. His personal hygiene was poor and his living quarters were filthy. He consumed large quantities of foods with high fat and sugar content, and became progressively more obese. He also displayed abnormal food choices, for instance, eating uncooked frozen foods. Given his frequent absences, tardiness and general lack of dependability, he could not hold a job. He showed reckless financial behaviour which resulted in large debts, and engaged in poorly planned petty thievery. He frequently threatened others and occasionally engaged in physical assault. He lied frequently, often without apparent motive. He had no lasting friendships and displayed little empathy. His sexual behaviour was irresponsible. He fathered a child in a casual relationship, and did not fulfil his paternal obligations. He was dependent on his parents for financial support and legal guardianship. He showed no guilt or remorse for his behavior

Both children came from stable middle class families, and had perfectly normal siblings.

The great difference between children with these problems and adults with similar injuries, or even "normal" sociopaths is that they are unable to grasp that social rules exist. A "normal" psychopath grasps very well that other people believe rules exist, and could tell you what they are. She just doesn't suppose that they apply to her.

The behaviour of our patients differed from the typical profile of psychopathy in that our patients' patterns of aggression seemed impulsive rather than goal-directed, and also in the highly transparent, almost child-like nature of their transgressions and their attempts to cover them.

Such people are incurable. It is not their fault. But they have missed the chance to learn the moral codes, the reciprocity, that make us fully human. Whereas the mind can work around some kinds of brain damage, it appears that this particular sort can't be replaced. Without certain areas of the frontal cortex, children will never learn social rules, nor even that social rules exist. In fact some of them, like the Doncaster torturers, will actively seek opportunities to cause pain.

I don't see how a fully consistent consequentialist can justify keeping them alive. I would keep them alive, because don't think we are, or should be, fully consistent consequentialists. But if we were, then it's hard to see what is the point of spending, over a lifetime, millions of pounds keeping such people locked up, or treating their illnesses, when they will never be cured and never become complete reciprocal members of society. If a brain scan could show that the parts of a child's brain needed to form a moral sense have been destroyed, don't we have a moral duty to spend the money on other children, with better brains, who could be helped and changed by treatment?

If an unborn foetus need not be kept alive because its brain is not fully developed, then why should a brain-damaged adult be special, especially when that damage destroys their moral sense? And what about old people, whose brains have been destroyed by Alzheimers? Martin Amis can see no logical reason to keep them alive. Can you?